for this to work your mslug4 rom must be around 30 mb (that's right no kidding) , and the file MUST be renamed to mslug4nd
alright first of all forget nebula. it's the worst arcade emulator that nobody uses. what you need is a hack of kawaks emulator. you can find that in this website:
http://usuarios.lycos.es/neogeoweb/emuladores.htm it's the first emulator in the table. The file is password protected. The password is
www.neogeoworld.tk
now put your mslug4 zip (renamed to mslug4nd) in the rom directory of the emulator. also put the neogeo bios in the same folder (the neogeo bios should be in a zipped folder named neogeo.zip) If you don't have the neogeo bios go to
www.google.com and type neogeo.zip or neogeo bios, you should get the desired file.
Now start the emulator. Go to File, Load Game, select Only available, make sure that Show Neo-Geo Games is checked, and finally click Rescan All. Metal Slug 4 should show up.
If your mslug4.zip file is bigger than 30 mb (70-80 mb) then i suggest you "shop" around until you find the 30 mb one.
Peace out!